Elective cryopreservation of all day 5 blastocysts is more effective than using day 6 blastocysts for improving pregnancy outcome in stimulated cycles.

To evaluate the efficacy of cryopreservation of all blastocysts for future transfers in stimulated cycles. We carried out fresh blastocyst transfer cycles on day 5 ( = 290) or day 6 ( = 119) and thawed blastocyst transfer cycles that were frozen on day 5 ( = 136), day 6 ( = 71) or day 6 electively ( = 21). We retrospectively compared the clinical outcome of fresh blastocyst transfers with thawed blastocyst transfers according to the day of blastocyst transfer or freezing. The clinical implantation rates in women with stimulated cycles were significantly higher after the transfer of thawed blastocysts compared with the transfer of fresh blastocysts (day 5,  < 0.0005; day 6,  < 0.00005). Although the implantation rate of fresh day 6 transfer cycles was lower than that of elective day 6 frozen-thawed cycles, this difference was not statistically significant ( = 0.17). Thawed blastocysts demonstrated a better potential for implantation when compared with fresh blastocysts in stimulated cycles. We concluded that elective cryopreservation of all blastocysts on day 5 is an effective option to improve the clinical outcome in stimulated cycles. Additionally, with cryopreservation of all day 6 blastocysts, the implantation rates of first embryo transfers may increase by allowing the best-quality blastocysts to be transferred in thawed cycles. (Reprod Med Biol 2008; : 75-83).